This invention relates to a method and apparatus for upshifting the frequency of photons and more particularly to a method and apparatus for upshifting the frequency of a pulse of electromagnetic radiation (EM wave) by propagating it through a gas which is being rapidly ionized to form a plasma. The frequency of the final radiation can be controlled by adjusting the gas density and hence the plasma density. Thus, a tunable source of radiation can be provided which spans a large frequency range, say, from microwaves to optical. Furthermore, by using a non-uniform gas density, chirped EM pulses with time-varying frequency can be created. Such chirped pulses can be created over a wide frequency range. Chirped pulses can be time-compressed to give very short intense radiation pulses.
With recent advances in laser technologies, high-power lasers capable of producing photon pulses of duration 10-1000 femtoseconds (fs) with photon energies of between 2 and 4 eV and total energies of 10.sup.-3 to 1 joule are coming to be available. With such a laser, a small region in space (from 1 mm.sup.3 to a few cm.sup.3) of a gas can be rapidly ionized within a time period on the order of the pulse duration of an EM wave propagating therethrough. It has been considered of interest to investigate the effects of such a rapidly created plasma on the EM wave propagating therethrough.